III.

Supernovae

A supernova explosion is far more spectacular and destructive than a nova and much rarer. Such events may occur no more than once every few years in our galaxy, but despite their increase in brilliance by a factor of billions, only a few are ever observable to the naked eye. Only four have been so observed in the past 1,000 years (1006, 1054, 1572, and 1604), although there is also evidence for a supernova taking place in the constellation of Cassiopeia around 1680. The best known of these was recorded by Chinese astronomers in ad 1054 and its remains are now known as the Crab Nebula. Supernovae, like novae, are more often seen in other galaxies. Thus, the most recent naked-eye supernova, which appeared in the southern hemisphere on February 24, 1987, occurred in the Large Magellanic Cloud, a companion galaxy to the Milky Way. This supernova, which exhibits some unusual traits, is now the object of intense astronomical scrutiny.

The mechanisms that produce supernovae are less well understood than those that result in novae. What is clear is that stars that are much more massive than the Sun sometimes explode in the late stages of their rapid evolution. This is caused when the star runs out of nuclear fuel in its core. This leads to a catastrophic gravitational collapse, since the pressure created by nuclear processes within the star is no longer able to withstand the weight of the star's outlying layers. This is called a Type II supernova.

A Type I supernova is brought about in a similar way to a nova. In the case of a Type IA supernova, a white dwarf that belongs to a binary system receives an influx of fresh fuel when it captures material from its swollen companion. When its mass becomes too great (that is, it passes the Chandrasekhar limit), the white dwarf collapses, causing a huge explosion. Type IB and IC supernovae show similar light curves to Type IA but tend to be dimmer. Type IB supernovae are thought to be produced by explosions in the cores of massive stars that have been stripped of their hydrogen. Type IC supernovae have been stripped of their helium.

Type I supernovae are used in cosmology to measure distance, and hence the age of the universe. Because their absolute magnitude and rate of brightening and dimming follow a consistent pattern, they can be used as “standard candles” to measure distance. Since distant objects in the universe are moving away from us (and the more distant they are the faster they are receding), light waves from them are progressively lengthened as they travel through the universe, thus appearing to move towards the red end of the electromagnetic spectrum. This red shifting can therefore also be used to estimate distance.

A 1998 study of type I supernovae at distances of up to 10 billion light years showed that many of the red shifts were greater than expected, indicating that the expansion of the universe is accelerating. Subsequent studies have confirmed these observations and shown that the expansion rate of the universe was decelerating until about 5 to 6 billion years ago, and then began speeding up. This acceleration is thought to be due to a repulsive force known as dark energy.

Little may remain after the explosion of a supernova except the expanding shell of gases. A famous example is the Crab Nebula. At its centre is a pulsar, or rapidly rotating neutron star. Supernovae are significant contributors to the interstellar material that forms new stars. It has been thought for some time that supernovae explosions are also responsible for the creation of oxygen and all of the heavier elements and their spread throughout the interstellar medium. This was confirmed by observations of a supernova in the Small Magellanic Cloud using the Chandra X-Ray Observatory.

In 2003 it was suggested that supernovae could be the source of long-duration gamma-ray bursts observed in extremely distant galaxies. Recent studies using the SWIFT spacecraft, launched in 2004, indicate that they may be particularly associated with Type IC supernovae. This is presumably because the presence of a hydrogen envelope around the collapsing core can block the emergence of a gamma-ray burst jet.